It has so far been carried out to react with a low-molecular compound the terminal hydroxyl group of polyethylene oxide obtained from a 3-membered ether or of polyether, such as polypropylene oxide, polyepichlorohydrin, etc. (including a copolymer thereof), in order to introduce a functional group other than a hydroxyl group into the terminal (J. Polym. Sci., Polym. Chem. Ed., 22, 341-352 (1984)). In the prior art process, one kind of functional group has been introduced. It has also been carried out using a polyether carrying a unreactive functional group (methoxy group) at one end and a hydroxyl group at another end to convert said terminal hydroxyl group into another functional group, in order to introduce a functional group into only one of the ends (Makromol. Chem., 182, 1379-1384 (1981)).